Unraveling the Influence of the Preexisting Molecular Order on the Crystallization of Semiconducting Semicrystalline Poly(9,9-di-n-octylfluorenyl-2,7-diyl (PFO)

Understanding the complex crystallization process of semiconducting polymers is key for the advance of organic electronic technologies as the optoelectronic properties of these materials are intimately connected to their solid-state microstructure. These polymers often have semirigid backbones and flexible side chains, which results in a strong tendency to organize/order in the liquid state. Therefore, crystallization of these materials frequently occurs from liquid states that exhibit—at least partial—molecular order. However, the impact of the preexisting molecular order on the crystallization process of semiconducting polymers— indeed, of any polymer—remained hitherto unknown. This study uses fast scanning calorimetry (FSC) to probe the crystallization kinetics of poly(9,9-di-n-octylfluorenyl-2,7-diyl (PFO) from both an isotropic disordered melt state (ISO state) and a liquid-crystalline ordered state (NEM state). Our results demonstrate that the preexisting molecular order has a profound impact on the crystallization of PFO. More specifically, it favors the formation of effective crystal nucleation centers, speeding up the crystallization kinetics at the early stages of phase transformation. However, samples crystallized from the NEM state require longer times to reach full crystallization (during the secondary crystallization stage) compared to those crystallized from the ISO state, likely suggesting that the preexisting molecular order slows down the advance in the latest stages of the crystallization, that is, those governed by molecular diffusion. The fitting of the data with the Avrami model reveals different crystallization mechanisms, which ultimately result in a distinct semicrystalline morphology and photoluminescence properties. Therefore, this work highlights the importance of understanding the interrelationships between processing, structure, and properties of polymer semiconductors and opens the door for performing fundamental investigations via newly developed FSC methodologies of such materials that otherwise are not possible with conventional techniques.

Wide-angle X-ray scattering (WAXS) experiments were measured simultaneously at beamline BL11 NCD-SWEET at ALBA Synchrotron Radiation Facility (Barcelona, Spain). FSC sensors were employed to place samples in the beam path. A THMS 600 Linkam hot stage device was employed for temperature control of the samples. WAXS diffractograms were recorded during crystallization of samples from ordered and disordered liquid states. Prior to WAXS analysis. The X-ray energy source amounted to 12.4 eV using a channel cut Si (1 1 1) monochromator (λ = 1.03 Å). The sample-detector distance was 132.6 mm with a 21.2° tilt angle, and chromium(III) oxide was employed to do the calibration (Rayonix LX255-HS detector, Evanston, IL, USA, with a resolution of 1920 × 5760 pixels and pixel size of 44 m 2 ). A PFO bulk sample was placed on top side of the chip. The sample was heated above the nematic-to-isotropic transition (T LC-I ) to erase the thermal history, then rapidly cooled (at 4,000 ºC/s) from the melt to the selected isothermal crystallization temperature, T a . Subsequently, the sample was kept at T a for 10h (the time it reaches maximum saturation), it was rapidly cooled to a temperature below T g , and rapidly heated to room temperature. Samples thus prepared were transported to the WAXS beamline.

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Polarized Light Optical Microscopy (PLOM) experiments were performed to observe the microscopic morphology of the material as a function of temperature. PLOM micrographs were taken at different temperatures during cooling from the melt at 60 °C/min. The representative micrographs are shown in Figure 1S. The micrograph shown in Figure 1SA was taken at a temperature above the clearing point at 300 °C, and as expected, a fully isotropic melt (ISO state) was observed, as no light can pass through the crossed polarizers.
As the sample is cooled, the nematic liquid crystalline state (NEM state) can be identified, as seen in Figure 1SB, where a weakly birefringent texture can be observed. When the temperature decreases to 80 °C, small crystallites in  Figure 1SC were observed, however, their distinction in the micrograph is rather difficult due to their size. This observation of the NEM state during cooling indicates that at a cooling rate of 60 °C/min, that is, the maximum cooling rate possible when using conventional PLOM and DSC techniques, the study of the kinetics of the material from a completely ISO state is not possible as the formation of the NEM state during cooling is unavoidable. Therefore, the conventional DSC technique is not suitable for this study and the use of FSC is the technique of choice, as this method enables to cool the sample at significantly faster rates.

Isothermal Crystallization Kinetics from the Isotropic and the Nematic Liquid States
The analysis of the heating curves after the isothermal crystallization for all temperatures are 6 given in Figure 2S.  Table S1. Overall crystallization rate parameters (experimental and obtained through the Avrami theory fit)

Isothermal Crystallization Kinetics from X-ray Scattering
In these experiments, the sensor with a previous thermal treatment done in the Flash DSC was placed perpendicular to the incident beam and in order to properly measure the sample and ensure a high S/N ratio, it was necessary to deposit the sample on the FSC sensor in the bulk, contrary to the FSC experiments which were measured in thin-films. The kinetics were followed by measuring the relative crystallinity from the peak increase at q = 15 nm −1 , indexed to the crystalline plane [530], with increasing time ( Figure 3S). The reflections found on the crystallized sample were able to be indexed to the orthorhombic unit cell of the α-crystal phase of PFO for both samples. 1 Results seen in Figure 3S reveal that for both methods (i.e., FSC and WAXS) similar experimental results are obtained for both crystallization protocols. In addition, after applying the Avrami theory, overall crystallization kinetics (k 1/n ) were significantly different for the two techniques. This could be simply explained since the analysis of the kinetics is done by following a single peak, that is, only one direction of the crystal growth has been analyzed. or due to different sample shape, that is in the bulk compared to thin-film. This signal is in the region of π-π stacking (010) so it is possible the stacking is perpendicular to the growth of the crystal and is inhibited in this direction and growing in another, which is not able to be observed in that direction. However, it was found that the Avrami index gives the same pattern for each initial crystallization state. That is, an Avrami index of 2 for an initial ISO state and 1 for an initial NEM state.
In Table 2S, comparisons between the two techniques are presented to outline comparabilities and differences in the results.

Fast Fourier Transform (FFT)
In FFT images of the phase of the AFM image, we are able to see a structure independently of its height. In the case of the image from ISO state, its FFT shows a vertically oriented ellipsoid. This shape is associated with globular domains in the real space with mean axis of around 30 nm (vertical) and 50 nm (horizontal). In the case of the NEM state, the FFT image represents elongated 11 domains with mean axis of 25 nm and 50 nm with two defined lobules associated with a correlation length of 50-60 nm, which is the double of the short axis. That means that the morphology of the NEM state is composed of correlated stackings of fibrillar domains, unlike the ISO state, in which the array of elongated domains do not show any correlation.